Modulation circuit



Aug- '28, 1945. A. ALFORD Erm. MoDULATIoN CRcUIT Filed Nov. 15. 194:5 2sheets-sheet 1,

kwxh

IN VEN TORS ANDREW Auro/ro Guy fr. P/:rf/Pso/v ATwRNEY 'Aug-28,1945. A.ALFORD ETA. f f 2,383,456

MODULATION CIRCUIT lated Wave.

Patented Aug. 28, 1945 FFICE y MODULATION CIRCUIT Andrew Alford,Cambridge, Mass., and Guy K.

Patterson, New York, N. Y., assignors to Federal Telephone and RadioCorporation,`NeWy ark, N. J., a corporation of Delaware ApplicationNovember 15, 1943, Serial No.510,268

12 Claims.

(Cl. Z50-11) This invention relates to modulation circuits and Imoreparticularly to modulation systems particularly suitable forenergization of radio beacons.

In certain types of radio beacons, it is often desirable to energize oneof the antenna units with carrierc and side band energy and to energizeother. cooperating antenna units with side band energy only. This hascommonly been done by systems feeding modulated energy to opposeddiagonal points of ahighfrequency bridge circuit of such a nature thatatoneterminal the carrier Wave 'will Vbalance out andl permit only sideband energy to be fed out,'wh'ile`lat the diagonally oppositeterminalthe carrier energies willv add in phase'so that both'` carrier and sideband output is obtained. i y

. Itis 'a principal object of our invention to provide'a modulatingcircuit which will produce separate modulated carrier Waves and sideband'wave outputs without the use of special bridge circuits.

It is afurther object of our invention to provide a modulating system inWhich voice frequency may be applied to a modulator simultaneously Withthe `application thereto of other modulating signals and carrierfrequency waves, and in which there is provided one output carrying thevoice frequency signals and the other modulating signalstogether withcarrier frequency, and a second output circuit carrying only the sidebands of said other modulating signal.

It is a further object of our invention to provide a small andlight-Weight modulating unit for energizing a radio beacon.

According to a feature of our invention, We pro'- vide a `vacuum tubearrangement operating su stantially as apusl'i-pull amplifiermodulatorto which unit is applied a radio frequency energy in push-pullrelation. Accordingly, in the output of the vacuum tube arrangement,there is provided an amplifiedV Vradio frequency excitation. Themidpoint of the push-pull output circuit'is coupled with-a furtheroutput loop tuned substantially to the'carrier frequency. However,because of the push-pull input and output characteristics, none of thecarrier frequency will appear in this circuit. A modulating signal isthen applied to-one grid only of the push-pull vacuumtube circuit, thusproducing an unbalanced modulated Wave in the o'utput'circuit of thevacuum tube arrangement. This unbalanced voltage will feed to the secondoutput circuit, and in that circuit will consistonly of the sidebandenergy 4of the modu- HoWever, in the principal circuit,

there will |be present the carrier frequency as 5,5

Well as the side. bandmodulating frequencies.` In order to preventintercoupling of the two output circuits, a shielding is generallyprovided bletfween the two output circuits. Furthermore, in orderprecisely to balance the push-pull modulation, independent bias controlis provided for the push-pull vacuum tube circuit, and in the outputcircuit, a differential phase balancingcondenser is provided.

A better understanding of our invention and the objects and featuresthereof may be had from the particular description thereof made Withreference Ato the accompanying drawings, in which:

Fig. a block schematic diagram of a radio beacon arrangementincorporating a modulating system in accordance With our invention; and

Fig. 2 is ya Schematic Wiring diagram ofthe modulating circuit inaccordance with `our in-y vention.

Turning first to Fig. 1, I0 represents a radio frequency source vcoupledto the input loop of modulator II. In the output of modulator II isprovided `a balanced output loop I3 coupled over a loop I4 to acentrallypositioned antenna unit I5. A modulating signal is suppliedfrom source I2 to one side of modulator II, producing inloop I3 amodulated carrier Wave which consists ofy the carrier frequency and theside band energy.V This energy is supplied to unit I5. Coupled to, themidpoint of loop I3.is a second output loop I6. This loop, because ofits symmetrical connection With respect to loop I3, Will not receive anyof lthe balanced radio frequency energy from source I0. However, themodulating signal is unbalanced in character and therefore there will bedeveloped in loop I6 side band energy. This energy is fed overa couplingline and a keyer I9 to the outer antenna units I1 and I8 arranged onopposite sideso-f central antenna unit I5. The respective lengths of thelines are so chosen that en'ergy in unit I5 is 90- in yphase fromenergyin units I'I and I8 While the energy inunits I'I and I8 is inphase opposition With respect to one another. Keyer I9 servesalternately to reverse the potential fed tok antenna units I'I and I8,thus prod-ucing a keyed beacon signal.

Should it be desired to also apply voice signal to the beacon, thismaybe :done by connecting a voice source 20 across the input loop ofmodulator- I I so that this voice energy is in parallel. Thus,

the radio frequency energy will 'be modulated so that this voicefrequency energy will be applied to central antenna unit I5 and toantennas II and I8.

,A preferred circuit for the modulator I I of Fig.v

l together with the input and output loops is illustrated in Fig. 2. Inthis arrangement, energy from `the radio frequency source is suppliedover a loop 2! to inputloop 22. This input loop applies the carrier wavein push-pull to the control grids 30, 4G of a double tetrode tube 23. Ifdesired, parasitic suppressing resistors 22a may be included in loop22.1 A condenser S0. is provided in loop 22 so that the bias on theseparate grids 39 and 4B may be independently controlled and further sothat signals may be independently applied to these grids. The balancedloop 'output circuit I3 tuned to resonance at the carrier frequency by avariable condenser 32 is coupled to anodes 3|, 4I 5of tube 23. A-t themidpoint 33 of loop I3 is provided a connection to loop I6 which istuned by variable condenser 34 and which may be connected to groundthrough radio frequency choke 35 and `by-pa'ss condenser 36. A loop I4is Vcoupled with loop I3 to furnish the output leads for this circuit.Energy from a modulating signal source I2 is applied over lead 46 andradio frequency choke 42 to grid 4I] of tube 23. This serves to apply anunbalanced modulating signal producing unbalanced sideband energy in`output loop I3, thus providing in I3 sideband energy as well ascarrierfrequency energy. The unbalanced sideband component in loop I3 isalso fed over midpoint 33 to loop circuit IB and from this circuitl issupplied over output leads 38 to a desired load which may be antennassuch as I1 and .I8 of Fig. l.

In order that the circuit may be properly balanced, we provide adifferential'bias control comprising potentiometers 48 and 49 connectedby leads 46, 4l and chokes42, 43, respectively, to grids4 and 30 0f tube23. A voltage regulating tube 50 is provided to assure that constantvoltage is applied across potentiometers 48 and 49. By adjusting thisdifferential bias control, an amplitude balance in the output of tube 23may be readily achieved. In addition, a differential phase balancingcondenser 55 is provided across tuned loop I3. This condenser isadjusted until -thephase balance isachieved in loop I3 so that in theabsence of modulating signal from source I2 there is no output in loopI3. By-pass condensers 44V and 45 may be provided to by-pass any radiofrequency energy which may pass chokes v42 and 43 tovground.

To prevent any coupling between the input and output lcircuits 22,4 I3and I5, separate shield arrangements I, 52 and 53 are provided abouteach of these separate components. This shielding is essential if allthe carrier frequency is to be excluded from loop I6 when the parts areassembled, for example, in a compact modulating unit where close spacingis required.v

In an actual full-scale operating model of the modulator in accordancewith this invention, it was found ypossible to make the entire unit insuch size to be readily portable. The unit, including the modulator, thenecessary radio frequency oscillators and the tone generating source,may be housed in a casing about the size of a suitcase and light enoughto be readily carried by one man.

While We havedescribed a particular embodiment of our invention relatedto a particulartype of beacon, it is' clear that this description ismade merely by way of example and that many lmodiiications may presentthemselves to those skilled in the art. For example, if it is desired tohave the voice frequency sidebands trans-- vmitted from loop I6 to theexclusion of loop I3, it is merely necessary to make the connections togrids 30, 4I) in parallel instead of pushpull relationship.

Also, instead of keying the output line, the keying device may bearranged to alternately connect source I2 to lines 46 and 41, ifdesired. What we consider as our invention is embodied in the objectsset out above and in the accompanying claims.

What is claimed is:

1. A modulating system for producing two output waves, one of carrierfrequency plus side bands, and the other of side band frequencies only,comprising vacuum tube means having two control grids and two anodes, aninput circuit coupled to said grids, a center-tapped loop output circuitcoupled tov said anodes, a source of carrier frequency energy, means forcoupling said source of carrier frequency energy to said control gridsin push-pull relation, a source of signal energy, and means for couplingsaid source of signal energy to one only of said control grids tolmodulate said carrier frequency energy to produce side bands, wherebycarrier frequency and side band energy is developed in said loop outputcircuit, and side band energy `only at said center tap.

2.'A system according to claim l, further comprising separate biasadjusting means coupledv to said, control grids toindependently adjustthe bias on said grids.

3. A system according to claiml, further comprising differentialcondenser means connected across said loop outputicircuit to adjust thebalance in said loop.

4. A modulating system according to claim l, further comprising biascontrol means for independently adjusting the bias of said controlgrids,

and a differential condenserv coupled across said loop output circuit toadjust the phase balance therein.

5. A modulating system according'to claim 1, further comprising aresonant circuit coupled kto the center tap of said loop output circuitfor receiving said side band `energy only, and shielded means forshielding said resonant circuit from said loop output circuit.

A6. -A modulating system accordingto claim 1,

further comprising voice signal means, and means for applying said voicesignals to said control grids in push-pull relation, whereby voicemodulated energy is developed in said loop output circuit but isbalanced out at said center tap.

7. In a radio beacon having a central radiating means, two sideradiating means arranged on opposite sides of said central radiatingmeans and a line interconnecting said side radiating means in phaseopposition, means for supplying carrier.

and side band energy to said central radiating means and substantiallyside-band energy only to said side radiating means, comprising vacuumtube means having two control grids and two anodes, an input circuitcoupled to said grids, a center tapped loop output circuit coupled tosaid anodes, a sour-ce lof carrier frequency energy, meansy for couplingsaid source of carrier frequency energy to said control grids inpush-pull relation, a source of signal energy,.means for coupling saidsource of signal energy to one only of said control grids to modulatesaid carrier frequency energy to produce side bands, whereby carrierfrequency and side band'energy is developed in said loop output circuit,and side band energy only in at said center tap, first coupling meansfor coupling said loop output circuit to said central radiating means, asecond output ycircuit connected to the center tap of said loop in saidlast named means comprises keying means for alternately reversing saidsecond coupling means. v

9. A modulating system according to claim 7,

-further comprising bias control means for independently adjusting thebias of said control grids, -an'd a differential condenser coupledacross said loop output circuit to adjust the phase balance therein.

10. A modulating system according to claim 7, further comprising voicesignal means, and means for applying said voice signals to said controlgrids in push-pull relation, whereby voice modulated energy is developedin said loop output circuit but is balanced'out at said center tap.

11. A modulating system according to claim 7, further comprising biascontrol means for independently adjusting the bias of said controlgrids, a differential condenser coupled across said loop output circuitto adjust the phase balance therein, and shielded means for shieldingsaid loop output circuit and said second output'circuits from oneanother.

12. A modulating system for producing two output waves, one of carrierfrequency plus side bands and the other of. side band frequencies only,comprising vacuum tube means having two control grids and two anodes, aloop output circuit coupled to said anodes, a second output circuitcoupled to the electrical midpoint of said electric loop circuit, asource of carrier frequency energy, means for coupling said source ofcarrier frequency energy to said control grids' in push-pull relation, asource of signal energy, and means for coupling said source of signalenergy to only one of said control grids to modulate said carrierfrequency energy to produce side bands, whereby carrier frequency sideband energy is developed in said loop output circuit and side bandenergy only in said second output circuit.

, ANDREW ALFORD.

GUY K. PATTERSON.

